Fix 41 of the 61 tests which fail with modules enabled: we were computing and

Fix 41 of the 61 tests which fail with modules enabled: we were computing and
caching the linkage for a declaration before we set up its redeclaration chain,
when determining whether a declaration could be a redeclaration of something
from an unimported submodule. We actually want to look at the declaration as if
it were not a redeclaration here, so compute the linkage but don't cache it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178733 91177308-0d34-0410-b5e6-96231b3b80d8